Karlite, Mgr(BOr)3 (OH,Cl)s a new borate mineral and associated ludwigite from the Eastern Alps

Karlite, a new borate mineral with the idealized formula Mgr(BOr)r(OH,Cl), occurs in a clinohumite-chlorite marble associated with ludwigite at Schlegeistal, Zillertaler Alpen, Austria. The mineral is white to light green and has a silky luster. It occurs as aggregates of minute needles and prisms elongated parallel to c, as much as l0 mm long, which sometimes have rosette-like forms. Karlite is biaxial negative with a mean 2v of 24o, a: 1.589, F: 1.632, y = 1.634, y d: 0.045, X: c, Y : b,Z: a; and is colorless in thin section. The (001) cleavage is perfect. The two most common forms are {ll0} and {100}. Hardness is 5.5. Streak is colorless. Karlite is orthorhombic, Y2r2,2r, with a: 17.929(5), b: 17.600(5), c: 3.102(l)A, Z:4, G*" : 3.02, G-.* : 2.80 to 2.85. The six strongest lines of the X-ray pattern (4 in A, intensity, hkl) are 2.21,100,810,740,441; 2.83,92,620;2.25,87,251; 2.7g,92,260,221,540; 12.53,76,11o; an.d2.89,68, 160,221,121. IR spectra indicate that water is present as (oH) only. DTA shows only one strong nonreversible endotherm at 830"C. Melting occurs at 1345"C. Combined microprobe, thermogravimetric and wet chemical analysis yielded SiO2 0.02, TiO2 < 0.01, B2O3 22.92, A12O32.96, Ct2O, < 0.01, FeO 2.27, MnO 0.17, MgO 58.90, CaO 0.14, KrO 0.01, NarO 0.01, F 0.16, Cl 3.26,H2O* 10.40, HrO0.63 (not included in total), total 101.22, O = F 0.82, corr. total 100.40. The mineral is named in honor of Prof. Dr. Franz Karl late professor of mineralogy and petrography at the Christian Albrechts University, Kiel, West Germany; in recognition of his geologic studies of the Eastern Alps.